This invention relates to microwave directional couplers and, more particularly, to microwave directional couplers which have a small physical size for a given operating frequency.
The demand for a smaller and lower cost components for consumer electronics has increasingly led to efforts to reduce the sizes of various microwave components. An example of such a component is a microwave directional coupler utilized in wireless terminals, such as cellular telephones, for monitoring transmitted power. In such applications, size and weight are critical parameters.
A conventional microwave directional coupler utilizes two 50 ohm transmission lines, each having an electrical length of one quarter wavelength at the operating frequency. The spacing between the transmission lines is selected to provide the desired electromagnetic coupling. At an operating frequency of 1.95 GHz, the length of a conventional microstrip directional coupler is 19 millimeters (mm). This dimension is large in proportion to the overall package size of typical wireless terminals.
Microstrip directional couplers having a capacitor or other reactive element connected between the two transmission lines are disclosed in U.S. Pat. No. 4,216,446, issued Aug. 5, 1980 to Iwer, and U.S. Pat. No. 5,159,298, issued Oct. 27, 1992 to Dydyk. The capacitor or other reactive element is stated to improve the directivity of the directional coupler.
A directional coupler having a capacitor connected between transmission lines and shunt capacitors connected between each transmission line and ground is disclosed in U.S. Pat. No. 5,243,305, issued Sep. 7, 1993 to D""Oro et al. The capacitors are connected at the center of the transmission lines and are stated to increase the directivity of the directional coupler.
A capacitively compensated microstrip directional coupler is disclosed in U.S. Pat. No. 4,999,593, issued Mar. 12, 1991 to Anderson. Reactive coupling networks are coupled between the transmission lines of the directional coupler at each end. Each reactive coupling network includes a first capacitor coupled between a common node and the first transmission line, a second capacitor coupled between the common node and the second transmission line, and a third capacitor coupled between the common node and ground.
All known prior art microwave directional couplers have had one or more drawbacks, including but not limited to unacceptable physical size and a large number of compensation components. Accordingly, there is a need for improved microwave directional couplers.
According to a first aspect of the invention, a microwave directional coupler is provided. The microwave directional coupler comprises a first transmission line having an input port and an output port, a second transmission line electromagnetically coupled to the first transmission line, the second transmission line having a coupled port and a terminated port, a first capacitor coupled between the input port and a reference potential, a second capacitor coupled between the output port and the reference potential, a third capacitor coupled between the coupled port and the reference potential, a fourth capacitor coupled between the terminated port and the reference potential, and a fifth capacitor coupled between the output port and the terminated port.
The first, second, third and fourth capacitors may have substantially equal values. The reference potential may comprise ground potential.
In some embodiments, the first and second transmission lines comprise microstrip transmission lines. In other embodiments, the first and second transmission lines comprise stripline transmission lines. The first and second transmission lines may comprise coplanar waveguide transmission lines or other transmission lines.
In some embodiments, the first, second, third, fourth and fifth capacitors may comprise lumped capacitors. In other embodiments, the first, second, third, fourth and fifth capacitors may comprise distributed capacitors. The first, second, third and fourth capacitors may each comprise an open circuit stub. The fifth capacitor may comprise a plurality of interdigitated conductors.
According to a further aspect of the invention, a method is provided for microwave directional coupling. The method comprises providing a first transmission line having an input port and an output port, and a second transmission line having a coupled port and a terminated port, positioning the first and second transmission lines for electromagnetic coupling between the transmission lines, coupling a first capacitor between the input port and a reference potential, coupling a second capacitor between the output port and the reference potential, coupling a third capacitor between the coupled port and the reference potential, coupling a fourth capacitor between the terminated port and the reference potential, and coupling a fifth capacitor between the output port and the terminated port.